Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined number of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or the air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy.
For example, a fuel cell tightening apparatus disclosed in Japanese Laid-Open Patent Publication No. 8-45535 is known. The fuel cell tightening apparatus is used at the time of stacking fuel cells to form a stack. As shown in FIG. 9, in the fuel cell tightening apparatus, two pairs of fuel cell stacks 3 are provided between upper and lower bolsters 1a, 1b, using an upper holder 2a, an intermediate holder 2b, and a lower holder 2c. A lower heat insulating block 4a is interposed between the lower holder 2c and the lower bolster 1b. An upper heat insulating block 4b, a plurality of springs 5, and bellows 6 are interposed between the upper holder 2a and the upper bolster la. Components between the upper and lower bolsters 1a, 1b are tightened together by tightening rods 7 and nuts 8.
According to the disclosure, with the above-described structure, it is possible to suppress variation of the tightening load due to the change in the spring constant or the decrease of the spring reaction force at high temperature as in the case of using only the springs 5, and it is possible to maintain the tightening load at a certain level by the springs 5 and the bellows 6.
However, in the conventional technique, since the springs 5 and the bellows 6 are used as a load mechanism, the structure of the load mechanism is complicated uneconomically. Further, the springs 5 are exposed to the high temperature environment during operation of the fuel cells. Therefore, the springs 5 are deteriorated easily.